Pre-combustion, in-combustion, and post-combustion techniques are used to capture carbon dioxide. In a pre-combustion carbon dioxide capturing method, various fossil fuels are partially oxidized (gasified) to produce a synthetic gas including hydrogen and carbon monoxide as main components, and the carbon monoxide is converted into hydrogen and carbon dioxide through a water gas shift reaction. Then, the hydrogen and the carbon dioxide are separated. Such a pre-combustion carbon dioxide capturing technique is used to capture carbon dioxide before a synthetic gas is used in various application fields (such as fuel cells, coal liquefaction, and compound production). In the pre-combustion carbon dioxide capturing technique, materials such as coal, biomass, and organic wastes may be used as raw materials instead of using petroleum which is relatively expensive and being exhausted, and produced synthetic gases may be used for various industrial fields such as power generation, fuel cells, synthetic material production. In addition, since carbon dioxide is collected at high temperature and high pressure, collection of carbon dioxide may less affect the efficiency of other processes, and carbon dioxide collecting costs may be markedly reduced because costs necessary for compressing carbon dioxide are reduced. Pre-combustion CO2 capturing techniques, such as a pressure swing adsorption (PSA) process and other common-use techniques using a physical sorbent such as Selexol and Rectisol, have low thermal efficiency due to low process temperature and requires a large amount of energy because of the necessity of a large amount of regeneration energy. Particularly, common-use wet process techniques require at least four steps such as two water gas shift steps, a heat exchange step, and a low temperature CO2 absorption step, and additionally, at least two compression steps for storing CO2 collected at low pressure. Furthermore, since the temperature of CO2-removed fuel gas is low, the fuel gas has to be reheated at a front side of a gas turbine, increasing costs and lowering efficiency. Techniques of using a separation film allow high-pressure operations and thus guarantee high energy efficiency. However, such techniques are not suitable for large-scale industrial processes because of low processing capacity.
A sorption enhanced water gas shift (SEWGS) technique may be used to effectively separate and capture CO2 from a synthetic gas generated by a gasifier while maintaining the CO2 at high temperature and high pressure. Since a CO2 collecting process can be performed together with a water gas shift (WGS) process by using the SEWGS technique, a CO conversion ratio may be improved, and since high-concentration CO2 can only be separated during a regeneration process by using the SEWGS technique, the SEWGS technique may be used as a pre-combustion CO2 capturing technique aimed to use pollution-free clean energy.
A fixed bed SEWGS technique for collecting CO2 after modifying natural gas has been developed in Europe. However, the technique is complex, requires up to seven processes, and is not suitable for continuous operation. Therefore, the technique has limitations for being used as a pre-combustion CO2 collecting technique for large-scale industrial processes such as an integrated gasification combined cycle (IGCC) process.
However, according to a fluidized bed SEWGS technique, conversion/absorption-regeneration is possible in a one-loop process, and collection of a large amount of CO2 is also possible.
According to the fluidized bed SEWGS technique, an absorbent and a catalyst are continuously circulated between two fluidized bed reactors to produce high-concentration hydrogen by simultaneously causing a CO conversion reaction and a CO2 capturing reaction in one of the reactors, and to separate high-concentration CO2 in the other of the reactors by applying steam and heat to the absorbent in which CO2 is captured to regenerate the absorbent. Since the catalyst and the absorbent are circulated continuously and repeatedly between the two reactors, the fluidized bed SEWGS technique is suitable for continuous operation and large-scale industrial processes such as an IGCC process. Since solid particles are used in the technique, waste water is little produced, and various inexpensive materials may be used owing to low corrosion. In addition, since absorbents can be regenerated and reused, the technique is attractive as future technology for collecting CO2 and producing hydrogen at low cost.
Such an SEWGS technique is disclosed in JP 3782311. In the disclosed technique, a catalyst including lithium silicate and a composite of iron oxide and chromium oxide are used, and methods such as a supporting method are used for preparing the catalyst. In U.S. Pat. Nos. 6,692,545 and 7,354,562, potassium carbonate, magnesium, manganese oxide, lanthanum oxide, or clay is proposed as an absorbent; an iron-chromium oxide catalyst is proposed as a high-temperature conversion catalyst; methods such as a supporting method are proposed as methods of preparing thereof. U.S. Pat. No. 7,083,658 proposes a potassium-oxide absorbent as a high-temperature absorbent but does not state about a catalyst, and JP 2000-262837 and JP 2005-041749 proposes various lithium-compound absorbents and iron-chromium oxide composite catalysts.
Recent technical papers relating to SEWGS are ChemSusChem., 2008, 1. 643-650, International Journal of Hydrogen Energy, 2009, 34, 2972-2978, Journal of New Materials for Electrochemicals Systems, 2008, 11, 131-136, International Journal of Greenhouse gas control I, 2007, 170-179, Journal of Power Sources, 2008, 176, 312-319, etc. The papers disclose research into optimal multi-step SEWGS processes using commercial low-temperature or high-temperature conversion catalysts (e.g., Sud-chemie) and absorbents prepared by adding additives to hydrotalcite containing magnesium and alumina.
Unlike the present disclosure, the above-mentioned patents and papers mainly propose: techniques for using commercial fixed bed catalysts or preparing fixed bed catalysts; combinations of various active materials, supports, and additives as absorbents; and preparing methods such as physical mixing methods and supporting methods. In addition, techniques proposed in such patents and papers are not suitable for preparing large amounts of catalysts and absorbents for fluidized bed processes, and are not suitable for processes in which a catalyst and an absorbent are continuously circulated between two fluidized bed reactors to collect CO2.